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Managing system processes
System processes, also known as 
daemons
, are programs that run in the background 
on your server and are typically started automatically when it boots. We don't 
usually manage these services directly as they run in the background to perform 
their duty, with or without needing our input. For example, if our server is a 
DHCP server and runs the 
isc-dhcp-server
 process, this process will run in the 
background, listening for DHCP requests and providing new IP assignments to 
them as they come in. Most of the time, when we install an application that runs 
as a service, Ubuntu will configure it to start when we boot our server, so we don't 
have to start it ourselves. Assuming the service doesn't run into an issue, it will 
happily continue performing its job forever until we tell it to stop. In Linux, services 
are managed by its 
init
 system, also referred to as 
PID 1
 since the 
init
 system of a 
Linux system always receives that PID.
In recent years, the way in which processes are managed in Ubuntu Server has 
changed considerably. Ubuntu has switched to 
systemd
 for its 
init
 system, which 
was previously Upstart until a few years ago. Ubuntu 16.04 was the first LTS release 
of Ubuntu with 
systemd
, and this is also the case today in Ubuntu 20.04, since we've 
had two LTS releases since then. Since 
systemd
 has been the standard for quite some 
time now, we'll focus our attention on the commands used with it to manage our 
services. Older 
init
 systems are ageing out.
With 
systemd
, services are known as 
units
, but to all intents and purposes, the terms 
"service," "daemon," and "unit" all essentially mean the same thing. Since I started 
using Linux over 18 years ago, I still refer to 
systemd
 units as services, out of habit. 
To help us manage these "units," 
systemd
 includes the 
systemctl
 command, which 
allows you to start, stop, and view the status of units on our server. To help illustrate 
this, I'll use OpenSSH as an example. 

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Controlling and Managing Processes